Read his ult's description. Consider what would happen if you're hitting/critting for very high damage, multiple times a second with 100% (or more!) lifesteal, then realize that it ALSO heals nearby teammates. I see so many people swear by AP Sion, but it just doesn't make as much sense to me. He doesn't carry that giant axe so he can bat his lashes at you with SUPER AP CRYPTIC GAZE! Sure, the exploding sheild is nice too. I recommend trying both builds out for yourself. If you wanna feel like a super tough nuker, go AP. If you wanna feel like an axe-wielding maniac with INSANE damage, life drain, and massive team healing, go AD.

For me, though, the choice is always obvious, and I vote AD. That ult is just toooo good.

I have tried it, and yes, it works. As long as you are attacking, you won't die. AD is for aggressive players who like to go all-in during attacks, since your damage doesn't come in burst, but in a longer drawn out fight where they lose health, and you don't.

Honestly, I've played both AD and AP builds on Sion and they both work out pretty well. There isn't a clear winner in my mind, though I personally prefer AD over AP. AP is too bursty for my taste. Of course, my AD build is more of an ASpeed build. Divine Blade + Stun + Ult + Malady = Fun. At that point, your only weakness is crowd control. Banshee's Veil and Cleanse more than make up for that. Golem buffs can make up for your early game mana deficiency, and help set you up for kills.